It is well known that diamond-like carbon (DLC) films offer a wide range of exceptional physical, mechanical, tribological, electrical, optical and biomedical properties that make them commercially attractive for numerous industrial applications. Certain DLC films are extremely hard, chemically inert and also tribologically they show some of the lowest known friction and wear coefficients. Their optical and electrical properties are extraordinary and can be tailored to meet the specific requirements of a given application. Because of their excellent chemical inertness, these films are resistant to corrosive and/or oxidative attacks in acidic and saline media. Main constituent of DLC is carbon (and also hydrogen, varies from few atomic percent to 60 percent) and because of its inertness property, it is also a biocompatible material. The combination of such a wide range of outstanding properties in one material is rather uncommon, so DLC can be very useful in meeting the multifunctional applications. These films are now used in numerous industrial applications, including razor blades, magnetic hard discs, critical engine parts, mechanical face seals, scratch-resistant glasses, invasive and implantable medical devices and microelectromechanical systems (MEMS). However, high residual stress (up to 10 GPa), low adhesion strength (especially with the metallic substrates) and low thermal stability of DLC films are major hindrance to limit their practical applications. However, these problems of DLC have been solved to an extent involving; compositional modification, multilayer architecture, interface modification etc. [A. A Voevodin, S. D. Walck, J. S. Zabinski, Wear, 203-204, 516, (1997); H. Dimigen and C. P. Klages, Surf. Coat. Technol., 49(1-3), 543, (1993); M. Gioti, S. Logothetidis and C. Charitidis, Appl. Phys. Lett., 73(2), 184, (1998); A.L. Baia Neto, R. A. Santo, F. L. Freire Jr., S. S. Camargo Jr., R. Carius, F. Finger, W. Beyer, Thin Solid Films 293(1-2), 206,(1997); J. C. Damasceno, S. S. Camargo Jr., F. L. Freire Jr., R. Carius, Surf. Coat. Technol. 133-134, 247, 2000)]
In a separate study some members of inventors have also suggested some possible ways to solve the problem of stresses in DLC films [Sushil Kumar, P. N. Dixit, D. Sarangi and R. Bhattacharyya, J. Appl. Phys. 85, 3866(1999); Sushil Kumar, D. Sarangi, P. N. Dixit, O. S. Panwar & R. Bhattacharyya, Thin Solid Films 346, 130 (1999)].
There are several excellent review articles written on DLC and other carbon based materials, which also emphasis that stress and adhesion are major limitation of DLC for realization of industrial applications [A. Grill, Surf. Coat. Technol. 94,  507(1997);H.C. Tsai and D. B. Bigy, J. Vac. Sci. Technol. 5, 3287(1987); A. Erdemir, O. L. Eryilmaz and G. Fenske, J. Vac. Sci. Technol. A 18, 1987(2000); J. Robertson Mater. Sci. Eng. 37, 129(2002)].
Most DLC films are structurally amorphous and the deposition methods and carbon sources used, the structural chemistry of the resultant films may also differ substantially, and such differences may, in turn, lead to large variations in their properties. DLC films are primarily made of sp2- and sp3-bonded carbon atoms that are extracted from carbon-containing sources, such as solid carbon targets, liquid or gaseous forms of hydrocarbons [J. Robertson, Mater. Sci. Eng. 37, 129(2002)]. The relative amount of sp2 versus sp3-bonded carbon atoms (essentially depends on energy of carbon ions while film formation) varies from one DLC films to another. Films with a high proportion of sp2-bonded carbon atoms tend to be relatively soft and behave more like graphite during tribological tests, while films with more sp3-bonded carbons are more like diamond, and hence they are hard and provide impressive tribological properties [J. P. Sullivan, T. A. Friedmann and K. Hjort, MRS Bull. 26,  309(2001)], are called tetrahedral amorphous carbon (ta-C, hydrogen free DLC). The atomic percent of hydrogen in these hydrogen free DLC is very low, normally less than one atomic percent. If the films are derived from a hydrocarbon source (using gaseous source, such as acetylene/methane or liquid source such as benzene/cyclohexane) then large amounts of hydrogen may also be present within their structures, such group of diamond-like carbon coatings is called hydrogenated amorphous carbon, a-C:H (H-DLC). The hydrogen content in these films is primarily independent variable that can differ considerably depending on the deposition method, hydrogen source gas and the deposition parameters used, which determines the structure and hence the properties of H-DLC. Incorporation of hydrogen in these films deposited by PECVD or sputtering, is generally required in order to obtain “diamond like” properties (reasonably hard, wide optical gap and high electrical resistivity) and to stablize the diamond structure by passivating dangling bonds by maintaining the sp3 hybridization configuration [C. Donnet and A. Grill, Surf. Coat. Technol. 94-95, 456 (1997); H. C. Tsai and D. B. Bogy, J. Vac. Sci. Technol. A 5, 3287 (1987); J. C. Angus and F. Jansen, J. Vac. Sci. Technol. A 6, 1778 (1988)]. The hydrogen content critically determines film structure at the atomic level and therefore, the physical properties of the films.
Compared with hydrogen-free DLC's (like ta-C), hydrogenated DLC's (like a-C:H) films are relatively soft but exhibit some of the lowest friction and wear coefficients. In particular, amorphous hydrogenated carbon (a-C:H, DLC) films have been extensively studied and have found a large number of applications [J. Robertson, Mat. Sci. Eng.R, 37(4-6), (2002), 129]. In general, ta-C films are more wear-resistant compared to the a-C:H films due to the higher hardness of ta-C. The wear of counterparts sliding against the ta-C coating is higher compared to the a-C:H coating due to higher surface roughness combined with the higher hardness of the ta-C coating. However, by doping the ta-C coating with hydrogen, the coefficient of friction reduced as the hydrogen content of the coating is increased. Friction properties of a-C:H and ta-C coating can be related to the friction properties of graphite, which verifies the importance of graphite formation and the availability of hydrogen for low friction properties of the a-C:H and ta-C coating. Thus, hydrogen is very important for obtaining diamond like properties.
In addition to H free DLC (a-C or to-C) and H-DLC (a-C:H), there exist several more DLC's consisting of different kinds of alloying elements, discrete compound phases in a nanocomposite and superlattice or nano-layered coating architectures. These DLC films are realized to combat residual stresses and adhesion strength of pure DLC. In particular, doping of DLC with certain metals, metalloids and gaseous species (such as Ti, B, S, Si, Cr, Ni, Ta, Cu, F, W and N) has become popular for various applications. The presence or absence of certain elemental species, such as hydrogen, nitrogen, silicon, tungsten, titanium and fluorine, in the microstructure of DLC can play significant role in deciding their properties.
There are several techniques (mainly PVD & CVD based) used to deposit DLC based hard and protective coatings on the outer surfaces of components. For irregular surfaces ion beam method is preferred or the components itself used as cathode. However, coating on inner surfaces remains a challenge and limited information is available in the literature. A hollow cathode plasma immersion ion processing method has been developed for the deposition of Si-DLC as hard and protective coating on the internal surface of stainless steel pipe [Boardman et al. US Patent Application, Pub. No. US2006/0011468 A1]. In another work, small hollow tube has been coated with Si-DLC [K. Nakanishi et al., Surface and Coating Technology, 2005; Yukimura et al. Surface and Coating Technology, 169-170, 4115(2003)].
Deposition of H free DLC on inner surface of cylindrical shaped hollows objects like bottle or tube seems to be complex. Carbon targets are used for the deposition of H-free DLC, as in any of the techniques like pulsed laser deposition (PLD), sputtering and vacuum arc. This makes the process very complex to create the plasma inside a bottle or tube. However, for this kind process, material has to be inserted inside the tube and then sputtering or arcing has to be made inside the tube [H. Wesemeyer, H. Veltrop, U.S. Pat. No. 5,026,466; W. F. Hensaw, J. R. White, A. Niiler, U.S. Pat. No. 4,407,712].
In addition, deposition of DLC on plastic surface has technological importance. Recently, the deposition of DLC on polymeric substrates has attracted considerable attention for various applications, like for plastic optics [A. Kimura, H. Kodama, T. Suzuki, J. Vac. Sci. Technol., A 21 (2003) 515.], for food packaging [Y. B. Guo, F. C. N. Hong, Diamond Relat. Mater. 12 (2003) 946.], for biomedical products [Y. Ohgoe, K. K. Hirakuri, K. Tsuchimoto, G. Friedbacher, O. Miyashita, Surf. Coat. Technol. 184 (2004) 263.] etc. Different polymeric substrates, such as Polycarbonate (PC), Polymethylmethacrylate (PMMA), Polyethylene terephthalate (PET) and Polyvinyl chloride (PVC) have already been coated with amorphous carbon films leading to a significant improvement of their chemical and mechanical surface properties [X. T. Zhou, S. T. Lee, I. Bello, A. C. Cheung, D. S. Chiu, Y. W. Lam, C. S. Lee, K. M. Leung, X. M. He, Surf Coat. Technol. 123 (2000) 273; N. K. Cuong, M. Tahara, N. Yamauchi, T. Sone, Surf Coat. Technol. 174-175 (2003) 1024; T. Tanaka, M. Yoshida, M. Shinohara, T. Takagi, J. Vac. Sci. Technol., A 20 (2002) 625]. ta-C, a-C:H, silicon doped tetrahedral amorphous carbon (ta-C:Si:H) and silicon doped hydrogenated amorphous carbon (a-C:H:Si) produced by PECVD and FCVA (Filtered Cathodic Vacuum Arc) techniques on PET and polycarbonate (PC) substrates have been investigated as gas barrier coating [G. A. Abbas, S. S. Roy, P. Papakonstantinous and J. A. McLaughlin, Carbon43, 303(2005)]. It has been observed that there is an improvement of oxygen barrier of PET film with DLC coating by plasma source ion implantation [M. Yoshida, T. Tanaka, S. Watanabe, M. Shinohara, J. W. Lee and T. Takagi, Surface and Coating Technology 174-175, 1033(2003)]. Two dimensional modeling and diagnostics of plasma for DLC deposition on PET has also been investigated [E. Amanatides, P. Gkotsis, C. Syndrevelis and D. Mataras, Diamond & Related Materials 15, 904(2006)]. Plasma Enhanced CVD has a definitive advantage for the growth of these films, because it permits the deposition on these sensitive polymer substrates even at room temperature [J. Robertson, Mater. Sci. Eng., R Rep. 37 (2002) 129]. DLC growth by plasma CVD technique is capable of a uniform deposition over large area on flexible substrates like plastic sheets. Polyethylene terephthalate (PET) is a useful plastic that can be blown into containers and bottles. PET has more flexibility in size and shape of packaging than metal cans. However, metal cans are easier to transport and handle than glass. Glass is breakable, heavy, and does not provide good portability. However, PET has a finite gas permeability that can limit the shelf life of some food and drink products. The addition of a gas-impermeable coating onto the inside wall of a standard PET bottle has long been considered as a way to improve the packaging for beer, fruit juice, and carbonated soft drinks, where the permeation of carbon dioxide, oxygen, and water needs to be suppressed. As these problems can be solved by using suitable coating on inner surface of plastic containers (like PET bottles); however, there are some essential specifications of coating required to solve the above mentioned problems. In this direction efforts have already been made as described in subsequent paragraphs.
There is an important invention which describes usages of pet containers for food and drinks containing recycled resin and having DLC coating deposited on inner surface (H. Kenichi and K. Tsuyoshi, US Patent Application 20030087030). In an another invention by other workers, describes about the amorphous carbon coating with polymer tendency on bottle or flask like container (D. David, R Jean-Micl, C. Patrick, B. Aima, B. Nasser and O. Fabrice, US Patent Application 20020179603). There is a joint patent by M/s Mitsubishi Shoji Plastics Corporation, M/s Hokkai Can Co., Ltd. and M/s Kirin Brewery Company Limited (M. Shigeki et al. U.S. Pat. No. 7,166,336,  JP1999000139211, JP1999000299806 and JP2000000048386) that describes about an apparatus for producing DLC film-coated plastic containers. In our invention outer electrode consists of a single electrode, however, in the said joined patent, in the apparatus, the outer electrode consists of three parts. The outer electrode unit comprises of a bottom electrode disposed along the bottom of the plastic container, and a body electrode disposed along the body of the plastic container, and the upper edge of the bottom electrode is positioned below the center between the top and the bottom of the plastic container. There are few more patents filed by same group about plastic containers coated with diamond-like carbon film (JP2001232714A2 and JP2007070734A2).
There is another patent which discusses about production of device for DLC film-coating on plastic container and production method thereof (H. Kenichi et al. U.S. Pat. No. 6,924,001, WO2000JP0009202, WO02051707A1, CN1479683A). They have also talked about plastic container for liquid medicine (EP1258234B1, WO0162202A1) and plastic container for liquid containing volatile organic substance (WO0162605A1,JP2000000048387). They have also obtained patent for plastic container for dry solid food (WO0162624A1, JP2000000048389). In another patent, H. Kenichi et al. (WO0132513A1) describes about the nitrogen-free DLC film coated plastic container, and method and apparatus for manufacturing the same. Further, in the same area a patent by H. Kenichi et al. (U.S. Pat. No. 7,029,752, JP2000000048388) describes about plastic container for liquid medicine and method of storing and recovering liquid medicine. Another patent (CN1207173C, JP2000000048387) describes about plastic container for liquid with diamond-like carbon film formed on inner surface. K. Ryota et al. (U.S. Pat. No. 5,922,418, JP1996000215557) describes about the method of forming a DLC film over the inner surface of guide bush.
In another patent (S. Osamu et al., U.S. Pat. No. 5,965,217, JP1996000266878) a method of forming DLC films over inner surface of cylindrical member is described. This particular patent relates to a method of forming a two-layer intermediate film over the inner surface of a cylinder, such as a bushing, with a DLC film being formed on the intermediate film with a uniform thickness by using an auxiliary electrode, which enhanced abrasion resistance of the inner surface.
In another patent (S. Eihaku U.S. Pat. No. 629,422, JP 1997000034508,  JP1997000061338) a method and apparatus for producing plastic container having carbon film coating is described.
In another patent (N. Kazufumi et al. U.S. Pat. No. 6,805,931, JP1994000189223, EP0773166B1) which describes about the plastic container coated with carbon film. This particular patent is about a plastic container having diamond like carbon film formed on the inner surface of the container made of plastic material. The thickness of the diamond like carbon film is within a range from 0.05 to 5 micron. The container coated with the diamond like carbon film can be used for a returnable bottle for beverage.
There is a European patent (N. Ikenaga EP1229068A1, JP2001000029176) which describes about method and apparatus for modifying the inner surface of containers made of polymeric compound. In this patent, DC high voltage pulses are applied from a high voltage power source to an electrode to implant ions into the interior side surface of the PET container so that the surface itself is modified into DLC.
In another patent (M. Akio, JP2007126746A2) describes about diamond-like carbon thin film as gas barrier formed on the surface plastic bottle.
A suitable coating must have sufficient hardness, relatively enough density; coating should be deposited at a high growth rate to make a viable industrial process, but without exerting a large heat load, which would soften the PET substrate (like in microwave process or high energy ions bombardment in ion assisted process); be biocompatible and food contact safe; and be optically tailorable (transparent or as required transparency; as some medicine/drugs need to be kept in brown/dark colored bottles). DLC, in principle, satisfies all these requirements. In addition, the optical bandgap can be varied over a wide range, so that it is possible to form an optically semitransparent coating on colored PET bottles, such as amber or green beer bottles, or brown/dark colored bottles, such as those used for medicine/drug bottles or a transparent coating for transparent bottles, such as those used for carbonated soft drinks or water. Ideally, ta-C would be the best material, but arc and PLD systems are not suited to coat the interior of plastic bottles. However, plasma system is ideal for this type of coating. Plasma enhanced CVD has a definitive advantage for the growth of these films, because it permits the deposition on these sensitive polymer substrates even at room temperature [J. Robertson, Mater. Sci. Eng., R Rep. 37 (2002) 129]. Thus, a-C:H is being used for this purpose by many groups [N. Boutroy, Y. Pernel, J. M. Rius, F Auger, H. J. von Bardeleben, J. L. Cantin, F. Abel, A. Zeinert, C. Casiraghi, A. C. Ferrari and J. Robertson, Diamond Relat. Mater. (2006) 15, 921 ; G. A. Abbas, S. S. Roy, P. Papakonstantinous and J. A. McLaughlin, Carbon 43, 303 (2005); M. Yoshida, T. Tanaka, S. Watanabe, M. Shinohara, J. W. Lee and T. Takagi, Surface and Coating Technology 174-175, 1033 (2003); S. Yamamoto, H. Kodama, T. Hasebe, A. Shirakura and T. Suzuki, Diamond Relat. Mater. (2005) 14, 1112]. Silicon oxide coating has also been used as gas barrier for improvement of transparent plastic films [T. Krug, Society of Vacuum Coaters, Proc. Of 33rd Annual Tech. Conf. 1990, P.163; T. Krug, R. Ludwig, G. Steiniger, Society of Vacuum Coaters, 36th Annual Tech. Conf. Proc. 1993, p. 302]. An interfacial layer between PET and silicon dioxide has also been used to overcome intrinsic brittleness of silicon dioxide which enabled the production of commercial level silica coated PET bottles [Toyo Seikan Kaisha, JP2003328131A2]. Toyo Seikan Kaisha Ltd., Toppan Printing Co. Ltd., and SIG Corpoplast GmbH Co. have developed silicon dioxide coating on inside of bottles using PECVD technique. The Coca-Cola Company has tried to coat these coating on outside of bottles [Coca-Cola Company, Patent No. WO0008226A2]. Microwave plasma system is in use in market to deposit a-C:H on inner surface of PET bottles [N. Boutroy, Y. Pernel, J. M. Rius, F Auger, H. J. von Bardeleben, J. L. Cantin, F. Abel, A. Zeinert, C. Casiraghi, A. C. Ferrari and J. Robertson, Diamond Relat. Mater. (2006) 15, 921]. M/s Sidel Inc. has developed DLC coating technology which they called as ‘Actis’ i.e. amorphous carbon treatment on internal surface [Sidel WO9949991A1]. Sidel process is basically a microwave-assisted process which excites the gas into plasma to deposit DLC layer on the inside of bottle. However, microwave systems and related components are not cost effective and one has to be careful about overheating of the plastic bottles.
A modified plasma based ion implantation (PBII) technique has also been used to deposit DLC coating on inner walls of PET bottles [M. Ikeyam, S. Miyagawa, Y. Miyagawa, Y. Hayakawa and T. Miyajima, Surface & Coating Technology 201,  8112(2007); M. Ikeyam, S. Miyagawa, S. Nakao, J. Choi and T. Miyajima, Nuclear Instruments and Methods in Physics Research B 257, 741 (2007)]. However, they have used high voltage pulse power for the generation of plasma which could impose some restriction to process. Very high power is used to get sufficient ion energy for the deposition of DLC films as in their process surface of bottle is in contact with ground potential and it is not a ion assisted process.
Kirin Brewery Co. Ltd. and Samco International, Inc. together have developed the first commercialized system for gas barrier DLC coating for PET bottles [Kirin Brewery, JP08053116A2; Kirin Brewery, Samco International JP2788412; A. Shirakura, M. Nakaya, Y. Koga, H. Kodama, T. Hasbe and T. Suzuki, Thin Solid Films 494, 84(2006)]. Since then many machine manufactures have made extensive efforts for the commercialization of DLC as gas barrier coating for PET bottles [E. Shimamura, K. Nagashima, A. Shirakura, Proceedings of 10th IAPRI Conference, Melbourne, 1997, p.251; The Coca-Cola Company WO0008226A2; Sidel WO9949991A1]. Kirin Brewery Co. Ltd. has got manufactured a commercial DLC coating machine by Mitsubishi Heavy Industries, Inc. having a capacity of 18000 bottles per hours [A. Shirakura, M. Nakaya, Y. Koga, H. Kodama, T. Hasebe and T. Suzuki, Thin Solid Films 494, 84 (2006)]. It is revealed from the description of machine that separate chamber has been used for accommodating bottles during mass production.
Drawback of the Prior Art
                1. There is limited scope for scale up for coating on inner surface of large number of bottles with proper Rf shielding, otherwise there is possibility of plasma discharge outside the plastic bottle during the deposition of DLC.        2. There is damage of inner surface of bottles due to ion bombardment on inner surface of bottle        3. High residual stress, low adhesion strength and low thermal stability of DLC films are major hindrances to limit the practical applications of the DLC coated bottle.        
To overcome all the aforementioned barriers there is a need to develop a more efficient process and apparatus for the deposition of DLC coating on inner surface of a shaped object.
Objects of the Present Invention
The main object of the present invention is to provide an improved process to deposit diamond like carbon film as protective coating on inner surface of a shaped object such as plastic or metallic bottles, which obviates at least one of the drawbacks of the prior art.
Another object of the present invention is to provide an apparatus for DLC coating on inner walls shaped object.
Yet another object is to provide a cost effective process and apparatus for DLC coating on inner walls of plastic bottles suitable for mass production.